In the drilling of wells in hydrocarbon exploration and development, it is common to drill boreholes that deviate significantly from the vertical. The drillbit in such deviated holes is controlled to make the trajectory of the wellbore follow a desired trajectory to intersect producing reservoirs at desired locations. Various surveying techniques are used to determine the position of the wellbore. In addition to inertial and gravity measurements, these techniques include determination of the direction of the earth's magnetic field.
The magnetic surveys are referenced to the magnetic north direction, i.e., the direction approximately defined by a magnetic compass. Most people incorrectly believe that a compass needle points to the north magnetic pole. But since the Earth's field is the effect of complex convection currents in the magma, which must be described as several dipoles, each with a different intensity and orientation, the compass actually points to the sum of the effects of these dipoles at any location. In other words, it aligns itself with the magnetic lines of force. Other factors, of local and solar origin, further complicate the resulting field. It may be all right to say that a compass needle points "magnetic north" but it only roughly points to the north magnetic dip pole.
Predictive geomagnetic models such as the World Magnetic Model (WMM) and the International Geomagnetic Reference Field (IGRF) only predict the values of that portion of the field originating in the deep outer core. In this respect, they are accurate to within one degree for five years into the future, after which they need to be updated. The Definitive Geomagnetic Reference Field (DGRF) model describes how the field actually behaved. Numerous publications give values of the magnetic declination using these models. These include the 1:39,000,000 Magnetic Variation chart of "The Earth's Magnetic Field" series published by the Defense Mapping Agency (USA), "The World Magnetic Variation 1995 and Annual Rates of Change" chart published by the British Geological Survey, and a 1:48,000,000 world declination chart of "The Magnetic Field of Earth" series that is published by the United States Geological Survey's Earth Sciences Information Center.
Local anomalies originating in the upper mantle, crust, or surface, distort the WMM or IGRF predictions. Ferromagnetic ore deposits; geological features, particularly of volcanic origin, such as faults and lava beds; topographical features such as ridges, trenches, seamounts, and mountains; ground that has been hit by lightning and possibly harboring fulgurites; cultural features such as power lines, pipes, rails and buildings; personal items such as crampons, ice axe, stove, steel watch, hematite ring or even a belt buckle, frequently induce an error of three to four degrees.
The stream of ionized particles and electrons emanating from the Sun, known as solar wind, distorts the Earth's magnetic field. As it rotates, any location will be subject alternately to the lee side, then the windward side of this stream of charged particles. This has the effect of moving the magnetic poles around an ellipse several tens of kilometers in diameter, even during periods of steady solar wind without gusts. The resulting diurnal change in declination is relatively small at tropical and temperate latitudes compared to the effect of local anomalies. For example, Ottawa is subject to plus or minus 0.1 degree of distortion. However; in Resolute, 500 kilometers from the north magnetic pole, the diurnal change cycles through at least plus or minus nine degrees of declination error. This error could conceivably be corrected, but both the time of day and the date would have to be considered, as this effect also varies with seasons.
The solar wind varies throughout an 11-year sunspot cycle, which itself varies from one cycle to the next. During severe magnetic storms, compass needles at high latitudes have been observed swinging wildly.
In typical downhole surveys, the position of the borehole is determined to within an accuracy of 3 ft per 1000 ft. of lateral displacement relative to the top of the borehole. Even a 1 degree error in the magnetic declination used in the survey makes an error of over 15 feet in lateral displacement at an offset of 1000 ft. This can be problematic in the case of wells drilled over a period of several years in the development of a field. In addition, leases including OCS lease sales periodically conducted by the United States Government involve the leasing of tracts designated in terms of geographic coordinates. It is important in the development of fields that any wellbore survey carried out using a magnetic reference be referred back to geographic directions and coordinates.
Magnetic borehole survey instruments measure direction with respect to the local magnetic field. All magnetic surveys are subject to errors if the horizontal component of the local magnetic field is not aligned with the local magnetic north. Errors of this kind may be caused by distortion of the local magnetic field by magnetic BHA materials in the borehole. Drillstring conveyed magnetic survey tools are usually run inside non-magnetic drill collars. However, it is not unusual for the measurement to be influenced by adjacent magnetic drillstring material. Therefore, frequent use is made of techniques to detect and subsequently remove errors due to drillstring magnetization. These techniques rely on the principle of having accurate earth's field data available, i.e. local earth's magnetic field strength and local earth's magnetic dip angle. Hence this invention also provides a technique to provide accurate local earth's field data to allow to correct the data due to magnetic interference.
The "Mag-01H declinometer/inclinometer" of Bartington Instruments comprises a single axis fluxgate magnetometer, with the magnetometer sensor mounted on a non-magnetic optical theodolite. As is noted in the description provided by Bartington Instruments:
"The magnetic axis of the sensor is aligned with the optical telescope of the theodolite and only records the strength of the component of the field along that axis. The theodolite is carefully leveled and the magnetic sensor axis is set to the horizontal position and rotated until a null is observed in the magnetic field. At this position the sensor is exactly normal to the earth's magnetic field and from the theodolite reading the direction of the field in the horizontal plane can be established. The true north direction is determined from solar or star observation or using a gyroscope, and the declination can then be calculated."
The necessity of establishing the true north direction from astronomical observations complicates the determination of magnetic declination. In addition, the theodolite observations noted above require human interaction while the use of a gyroscope has its own problems, particularly those associated with calibration and drift of the gyroscope.
The best navigational tool available for determining the position of an observation point on the surface of the earth is based on signals from Global Positioning System (GPS) satellites. Most GPS receivers have internal data and an algorithm to compute the declination after the position is established. The algorithm is based upon models such as the IGRF model and does not account for local and temporal variations. In any case, this data cannot be updated from satellite transmission, therefore it is subject to become outdated.
Accordingly, there is a need for an invention that determines the local Earth's magnetic field, local dip angle and declination at a borehole with a high degree of accuracy with a minimum of human intervention. Such an invention should preferably also have the capability of tracking the time-varying component of the magnetic field, i.e. local magnetic field, local dip angle and declination, whether it is caused by diurnal variation or by geomagnetic storms.